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DOI: 10.1002/anie.201202176
CO2 Capture
Polyamine-Tethered Porous Polymer Networks for Carbon Dioxide
Capture from Flue Gas**
Weigang Lu, Julian P. Sculley, Daqiang Yuan, Rajamani Krishna, Zhangwen Wei, and
Hong-Cai Zhou*
One of the most pressing environmental concerns of our age
is the escalating level of atmospheric CO2, which is largely
correlated to the combustion of fossil fuels. For the foreseeable future, however, it seems that the ever-growing energy
demand will most likely necessitate the consumption of these
indispensable sources of energy. Carbon capture and sequestration (CCS), a process to separate CO2 from the flue gas of
coal-fired power plants and then store it underground, has
been proposed to reduce the anthropogenic CO2 emissions.
Current CO2 capture processes employed in power plants
worldwide are post-combustion “wet scrubbing” methods
involving the chemical adsorption of CO2 by amine solutions
such as monoethanolamine (MEA). The formation of carbamate from two MEA molecules and one CO2 molecule
endows the scrubber with a high capacity and selectivity for
CO2. However, this process suffers from a series of inherent
problems, such as high regeneration costs that arise from
heating the solution (ca. 30 % of the power produced by the
plant), fouling of the equipment, and solvent boil-off.[1]
To sidestep the huge energy demand, corrosion problem,
and other limitations of traditional wet scrubbers, intensive
efforts have been made to investigate the use of solid
adsorbents as an alternative approach.[2] Compared to wet
scrubbing, in which a large amount of water (70 % w/w) must
be heated and cooled during the regeneration of the dissolved
amines, the solid adsorbent approach has the tremendous
advantage of improving the energy efficiency of the regeneration process by eliminating the need to heat water.
Porous materials, such as MOF-210,[3] NU-100,[4] and
PPN-4,[5] have been deemed to be viable storage alternatives
[*] Dr. W. Lu, J. P. Sculley, Z. Wei, Prof. Dr. H.-C. Zhou
Department of Chemistry, Texas A&M University
College Station, TX 77842 (USA)
E-mail: zhou@mail.chem.tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/zhou/
Prof. Dr. D. Yuan
Fujian Institute of Research on the Structure of Matter Chinese
Academy of Sciences
Fuzhou, 350002 (P.R. China)
Prof. Dr. R. Krishna
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
[**] This work was supported by the U.S. Department of Energy (DOE
DE-SC0001015 and DE-AR0000073) and the Welch Foundation (A1725). We acknowledge Dr. Vladimir Bakhmoutov for his help with
solid-state NMR spectroscopy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202176.
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because of their high porosity and, therefore, significantly
increased accessible contact area with gas molecules. This
could be advantageous because separation and regeneration
could be performed under relatively mild conditions compared to amine wet scrubbing systems. Unfortunately, the
record high storage capacities do not translate to high
selectivities and only moderate CO2-uptake capacities were
observed under carbon capture conditions.
The polarizability and large quadrupole moment of CO2
can be taken advantage of by introducing CO2-philic moieties
that create strong interactions between the material surface
and the CO2. This will improve the loading capacities and
selectivity of CO2 over other gases. Indeed, this approach has
already been proven to be very successful in enhancing the
enthalpy of CO2 adsorption,[6] which can be calculated from
CO2 sorption isotherms at different temperatures and used to
quantify the interaction between the material and CO2. It is
worth pointing out that the porosity of the material will be
compromised by the introduction of functional groups. CO2
loading capacities at ambient conditions are dependent on the
adsorption enthalpy and porosity (both surface area and pore
volume), which must be balanced to achieve high loading.
Besides the loading capacity, another important factor
when quantifying how well a material will perform is CO2
selectivity over N2. Taking amine scrubbing as the model,
aminated porous materials that have been synthesized usually
exhibit very large adsorption enthalpies for CO2 and high
CO2/N2 selectivities. Recently, Long and co-workers demonstrated the incorporation of N,N-dimethylethylenediamine
(mmen) at exposed metal centers of CuBTTri (H3BTTri =
1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene), with the new compound mmen-CuBTTri showing drastic enhancement of CO2
uptake at low pressure, as well as CO2/N2 selectivity. It has the
highest heat of adsorption for CO2 (96 kJ mol 1) and one of
the highest CO2/N2 adsorption selectivities (SIAST = 327 at
25 8C) reported to date.[7] Despite its large isosteric heat of
CO2 adsorption, mmen-CuBTTri could easily be regenerated
at 60 8C.
It is always important to keep physicochemical stability in
mind for practical applications. Purely organic porous polymers are a class of adsorbents that exhibit surface areas
comparable to those of metal–organic frameworks (MOFs),
but have much higher physicochemical stability because of
the entirely covalently bonded network.[6a, 8]
The key to obtaining porous polymers with high amine
loading is to judiciously select efficient reactions and starting
materials with ultrahigh surface areas. PPN-6[9] (PPN stands
for porous polymer networks), which has an exceptionally
high surface area and an extremely robust all-carbon scaffold
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based on biphenyl rings, is ideal for the introduction of CO2philic groups under harsh reaction conditions. PPN-6 (also
known as PAF-1, first reported by Ben et al.[8a]) was synthesized by using a modified Yamamoto homocoupling polymerization procedure.[10] It has a Brunauer–Emmett–Teller
(BET) surface area as high as 4023 m2 g 1, and takes up
1.3 mmol g 1 (5.4 wt %) CO2 at 295 K and 1 bar. The polyamine-tethered PPNs (Scheme 1, and see Experimental
Section) show dramatic increases in CO2-uptake capacities
at low pressures, and exceptionally high CO2/N2 adsorption
selectivities under ambient conditions.
Nitrogen gas adsorption/desorption isotherms were collected at 77 K (Figure 1 a). The calculated BET surface areas
were 1740, 1014, 663, 634, and 555 m2 g 1 for PPN-6-CH2Cl,
PPN-6-CH2EDA, PPN-6-CH2TAEA, PPN-6-CH2TETA, and
PPN-6-CH2DETA respectively. Notably, the huge adsorption/
desorption hysteresis in PPN-6-CH2Cl disappeared into
a nearly type I isotherm for the polyamine-tethered PPNs.
Along with the decrease in surface area, the pores are smaller
after tethering, which supports the reaction occurring within
the cavities (see Figure S4 in the Supporting Information).
The efficiency of amine substitution was confirmed by
elemental analysis (see Table S5 in the Supporting Informa-
Scheme 1. Synthetic route to polyamine-tethered PPNs. a) CH3COOH/
HCl/H3PO4/HCHO, 90 8C, 3 days; b) amine, 90 8C, 3 days.
Figure 1. a) N2 adsorption (*) and desorption (*) isotherms at 77 K. b) CO2 adsorption (*) and desorption (*) isotherms, as well as PPN-6CH2DETA N2 adsorption, at 295 K. c) The component loadings of N2 and CO2 calculated by IAST with bulk gas-phase partial pressures of 85 kPa
and 15 kPa for N2 and CO2, respectively, with PPN-6-CH2DETA, PPN-6-CH2EDA, PPN-6-CH2Cl, PPN-6-SO3Li,[9] NaX zeolite,[9] MgMOF-74,[10] mmenCuBTTri.[7] Loadings for calculated selectivities of 200 and 400 are shown as a guide. d) Isosteric heats of adsorption Qst for the adsorption of
CO2, calculated using the dual-site Langmuir isotherm fits.
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tion). The chlorine content of 14.42 % in PPN-6-CH2Cl was
reduced to only trace amounts in the polyamine-tethered
PPNs. PPN-6-CH2DETA had the highest nitrogen content of
11.95 % (see Table S5 in the Supporting Information), which
corresponds to a loading of 0.3 functional groups per phenyl
ring.
The introduction of polyamine groups in PPN-6 resulted
in materials with excellent CO2 adsorption characteristics at
295 K and low pressures (Figure 1 b). The trend of improvement in CO2 adsorption in terms of tethered polyamine
groups was DETA > TAEA > TETA > EDA. Although
PPN-6-CH2DETA has the lowest surface area, it exhibits
the highest CO2-uptake capacity of all the polyaminetethered PPNs. This finding indicates that the CO2-uptake
capacity is closely correlated to amine loading instead of
surface area under these conditions, and the result is
consistent with previous findings of Dawson et al.[8f] At
295 K and 1 bar, PPN-6-CH2DETA exhibits exceptionally
high CO2 uptake (4.3 mmol g 1, 15.8 wt %), possibly because
of the accessibility of the alkylamine chain. To the best of our
knowledge, this value is one of the highest of all the
microporous organic polymers reported so far,[8f, 9] including
top-performing N-containing ones, such as N-TC-EMC[11]
(4.0 mmol g 1), BILP-4[12] (3.6 mmol g 1), and MFB-600[13]
(2.25 mmol g 1).
Coal-fired power plants emit flue gas that contains
approximately 15 % CO2 at total pressures of around 1 bar;
thus, CO2-uptake capacity at about 0.15 bar (partial pressure
of CO2 in flue gas) is more relevant to realistic postcombustion applications. At 295 K and 0.15 bar, PPN-6CH2Cl only takes up 0.25 mmol g 1 CO2 (1.1 wt %), whereas
PPN-6-CH2DETA takes up 3.0 mmol g 1 of CO2 (11.8 wt %).
This latter value is comparable to other top-performing
materials, such as mmen-CuBTTri[7] (9.5 wt % at 298 K) and
MgMOF-74[14] (22.0 wt % at 293 K), but PPN-CH2DETA
stands out with respect to its physicochemical stability arising
from the covalent bonding in the framework. The increase in
the volumetric uptake capacity upon tethering of the polyamine chain is even more significant: from 1.3 g l 1 for PPN6-CH2Cl to 37.5 g l 1 for PPN-6-CH2DETA at 295 K and
0.15 bar (the tap densities were measured to be 0.12 and
0.28 g cm 3 for PPN-6-CH2Cl and PPN-6-CH2DETA, respectively; see Table S6 and Figure S2 in the Supporting Information).
At 295 K, the polyamine-tethered PPNs adsorb less N2
than PPN-6-CH2Cl (see Figure S3 in the Supporting Information). PPN-6-CH2DETA is especially interesting, with an
uptake of less than 0.1 wt % N2 at 1.0 bar, approximately one
third of that of PPN-6-CH2Cl. It is possible that the added
polar sites in the former may enhance N2 adsorption, but that
this is totally offset by the significant loss of surface area.
Conversely, the CO2 uptake is significantly enhanced because
the strong interactions play a more significant role.
To evaluate the CO2/N2 selectivity of the materials under
flue-gas conditions with single-gas isotherms, we used the
ideal adsorbed solution theory (IAST) model of Myers and
Prausnitz[15] along with the pure component isotherm fits to
determine the molar loadings in the mixture for specified
partial pressures in the bulk gas phase. Figure 1 c presents the
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component loadings of N2 and CO2 for PPN-6-CH2Cl, PPN-6CH2EDA, PPN-6-CH2DETA, PPN-6-SO3Li, MgMOF-74,
NaX zeolite, and mmen-CuBTTri calculated by IAST with
bulk phase equilibrium partial pressures of 85 kPa (N2) and
15 kPa (CO2). Two factors that define a material for efficient
separation of CO2 are a high CO2 loading and a high
selectivity over N2. Figure 1 c clearly shows both factors, and
highlights the high loading on PPN-6-CH2DETA and its
exceptional selectivity of CO2 over N2 (the better the
material, the closer to the top left corner of the graph). We
note that CO2 loading is highest for MgMOF-74; however,
PPN-6-CH2DETA has a much lower N2 uptake than
MgMOF-74, thereby leading to an unprecedented selectivity
compared to any reported material (see Table S7 in the
Supporting Information). The poor N2 adsorption with PPN6-CH2DETA can be attributed to low porosity (pore volumes
are 0.573 and 0.264 cm3 g 1 for MgMOF-74[14] and PPN-6CH2DETA, respectively).
To better understand the adsorption properties, the
isosteric heats of adsorption (Qst) were calculated from
dual-site Langmuir fits of the CO2 isotherms at 273, 284,
and 295 K. Figure 1 d shows a plot of the adsorption
enthalpies as a function of loading. Particularly noteworthy
are the strong inflections for all the polyamine-tethered PPNs.
As can be seen from Figure 1 d, PPN-6-CH2DETA retains its
strong CO2 interactions to a relatively high CO2 loading
(ca. 2.0 mmol g 1), which is consistent with its large CO2uptake capacity. The temperatures of flue-gas streams emitted from power plants, however, are much higher than 295 K.
To be practically useful for CO2 capture under realistic fluegas conditions, the preservation of high isosteric heats of
adsorption is essential for solid adsorbents so as to maintain
high CO2-uptake capacity at elevated temperatures. Assuming full regeneration, the quantity adsorbed at 0.15 bar could
be considered as the full capacity. From 295 K to 313 K, the
CO2-uptake capacity of PPN-6-CH2DETA at 0.15 bar only
dropped slightly from 11.8 to 10.0 wt % (see Figure S5 in the
Supporting Information); thus, this porous material holds
considerable promise for realistic post-combustion carboncapture applications.
The advantages of solid adsorbents with high heats of
adsorption include high selectivity and large CO2 capacities at
low partial pressures; however, concerns about material
regeneration are often raised. To test the cyclability of PPN6-CH2DETA we simulated temperature and vacuum swings
with an ASAP2020 analyzer, by saturating with CO2 up to
1.1 bar at 273 K followed by a high vacuum for 100 min at
80 8C. After 20 cycles, there was no apparent loss in capacity
(Figure 2), thus indicating complete desorption during each
regeneration cycle. Compared to mmen-CuBTTri, PPN-6CH2DETA shows higher energy demands for regeneration,
possibly because of the large percentage of primary amines in
PPN-6-CH2DETA as opposed to only secondary amines in
mmen-CuBTTri—primary amines have been shown to form
stronger bonds with CO2. Despite the high adsorption
enthalpies of these materials, the regeneration energies
would still be substantially lower than for amine solutions,
in which the chemisorption interactions (ca. 50–100 kJ mol 1)
necessitate heating the solutions containing about 70 % water
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flask was charged with PPN-6-CH2Cl (200 mg) and diethylenetriamine (DETA, 20 mL). The flask was sealed and heated to 90 8C for
3 days. The resulting solid was collected, washed with water and
methanol, and then dried in vacuo to produce PPN-6-CH2DETA as
a brown powder in quantitative yield.
Received: March 19, 2012
Published online: June 19, 2012
.
Keywords: adsorption · amines · CO2 capture ·
environmental chemistry · porous polymer networks
Figure 2. Twenty cycles of CO2 uptake at 273 K. After saturation, the
sample was regenerated with a temperature swing to 80 8C and then
under vacuum (0.1 mmHg) for 100 min. Data were collected with an
ASAP2020 analyzer.
to around 100 8C. In the latter case, the energy consumption is
directly linked to the high heat capacity of water
(4.15 J g 1 K 1).
Compared to aqueous alkanolamines, which not only boiloff but also degrade below 140 8C, higher decomposition
temperatures were observed when these amines were fixed
into PPNs. Thermogravimetric analysis (TGA) suggests
polyamine-tethered PPNs to be thermally stable in air up to
200 8C (see Figure S6 in the Supporting Information).
Physicochemical stability, high CO2 capacity, high selectivity, and low regeneration costs are the criteria by which new
adsorbents for post-combustion CO2 capture will be evaluated. The results presented herein demonstrate that aromatic chloromethylation of PPN-6 and a subsequent polyamine substitution bestows the materials with significantly
enhanced CO2-uptake capacity but much lowered N2-uptake
capacity at low pressures. PPN-6-CH2DETA exhibits an
exceptionally high binding affinity and the largest selectivity
for CO2 of any porous material reported to date. Given the
outstanding physicochemical stability and mild regeneration
requirements, this material has great potential for practical
application in a post-combustion CO2 capture technology.
Experimental Section
Synthesis of PPN-6: Tetrakis(4-bromophenyl)methane (205 mg,
0.32 mmol) was added to a solution of 2,2’-bipyridyl (226 mg,
1.45 mmol), bis(1,5-cyclooctadiene)nickel(0) ([Ni(cod)2]; 400 mg,
1.45 mmol), and 1,5-cyclooctadiene (cod; 0.18 mL, 1.46 mmol) in
anhydrous DMF/THF (30 mL/30 mL), and the mixture was stirred
overnight at room temperature under argon. 6 m HCl (20 mL) was
added, and the resulting mixture was stirred for 6 h. The precipitate
was collected by filtration, then washed with methanol and water, and
dried in vacuo to produce PPN-6 as an off-white powder (85 mg,
85 %).
Synthesis of PPN-6-CH2Cl: A resealable flask was charged with
PPN-6 (200 mg), paraformaldehyde (1.0 g), glacial AcOH (6.0 mL),
H3PO4 (3.0 mL), and conc. HCl (20 mL). The flask was sealed and
heated to 90 8C for 3 days. The resulting solid was collected, washed
with water and methanol, and then dried in vacuo to produce PPN-6CH2Cl as a brown powder in quantitative yield.
General procedure for the introduction of polyamine groups
(with the synthesis of PPN-6-CH2DETA as an example): A resealable
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[1] a) J. D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R. D.
Srivastava, Int. J. Greenhouse Gas Control 2008, 2, 9 – 20; b) N.
MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G.
Jackson, C. S. Adjiman, C. K. Williams, N. Shah, P. Fennell,
Energy Environ. Sci. 2010, 3, 1645 – 1669; c) T. Lewis, M. Faubel,
B. Winter, J. C. Hemminger, Angew. Chem. 2011, 123, 10360 –
10363; Angew. Chem. Int. Ed. 2011, 50, 10178 – 10181.
[2] a) Y.-S. Bae, R. Q. Snurr, Angew. Chem. 2011, 123, 11790 –
11801; Angew. Chem. Int. Ed. 2011, 50, 11586 – 11596; b) J.-R.
Li, R. J. Kuppler, H.-C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 –
1504; c) J.-R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H.-K.
Jeong, P. B. Balbuena, H.-C. Zhou, Coord. Chem. Rev. 2011, 255,
1791 – 1823; d) D. M. DAlessandro, B. Smit, J. R. Long, Angew.
Chem. 2010, 122, 6194 – 6219; Angew. Chem. Int. Ed. 2010, 49,
6058 – 6082; e) Q. Wang, J. Luo, Z. Zhong, A. Borgna, Energy
Environ. Sci. 2011, 4, 42 – 55; f) N. Hedin, L. Chen, A.
Laaksonen, Nanoscale 2010, 2, 1819 – 1841; g) T. C. Drage,
C. E. Snape, L. A. Stevens, J. Wood, J. Wang, A. I. Cooper, R.
Dawson, X. Guo, C. Satterley, R. Irons, J. Mater. Chem. 2012, 22,
2815 – 2823.
[3] H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi,
A. . Yazaydin, R. Q. Snurr, M. OKeeffe, J. Kim, O. M. Yaghi,
Science 2010, 329, 424 – 428.
[4] O. K. Farha, A. zgr Yazaydın, I. Eryazici, C. D. Malliakas,
B. G. Hauser, M. G. Kanatzidis, S. T. Nguyen, R. Q. Snurr, J. T.
Hupp, Nat. Chem. 2010, 2, 944 – 948.
[5] D. Yuan, W. Lu, D. Zhao, H.-C. Zhou, Adv. Mater. 2011, 23,
3723 – 3725.
[6] a) R. Dawson, D. J. Adams, A. I. Cooper, Chem. Sci. 2011, 2,
1173 – 1177; b) A. Demessence, D. M. DAlessandro, M. L. Foo,
J. R. Long, J. Am. Chem. Soc. 2009, 131, 8784 – 8786; c) A.
Torrisi, R. G. Bell, C. Mellot-Draznieks, Cryst. Growth Des.
2010, 10, 2839 – 2841; d) R. Babarao, S. Dai, D.-e. Jiang,
Langmuir 2011, 27, 3451 – 3460; e) A. Thomas, Angew. Chem.
2010, 122, 8506 – 8523; Angew. Chem. Int. Ed. 2010, 49, 8328 –
8344.
[7] T. M. McDonald, D. M. DAlessandro, R. Krishna, J. R. Long,
Chem. Sci. 2011, 2, 2022 – 2028.
[8] a) T. Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu,
F. Deng, J. M. Simmons, S. Qiu, G. Zhu, Angew. Chem. 2009, 121,
9621 – 9624; Angew. Chem. Int. Ed. 2009, 48, 9457 – 9460; b) W.
Lu, D. Yuan, D. Zhao, C. I. Schilling, O. Plietzsch, T. Muller, S.
Brse, J. Guenther, J. Blmel, R. Krishna, Z. Li, H.-C. Zhou,
Chem. Mater. 2010, 22, 5964 – 5972; c) J.-X. Jiang, A. Laybourn,
R. Clowes, Y. Z. Khimyak, J. Bacsa, S. J. Higgins, D. J. Adams,
A. I. Cooper, Macromolecules 2010, 43, 7577 – 7582; d) J. R.
Holst, E. Stçckel, D. J. Adams, A. I. Cooper, Macromolecules
2010, 43, 8531 – 8538; e) J. Germain, J. M. J. Frechet, F. Svec,
Chem. Commun. 2009, 1526 – 1528; f) R. Dawson, E. Stockel,
J. R. Holst, D. J. Adams, A. I. Cooper, Energy Environ. Sci. 2011,
4, 4239 – 4245; g) A. Trewin, A. I. Cooper, Angew. Chem. 2010,
122, 1575 – 1577; Angew. Chem. Int. Ed. 2010, 49, 1533 – 1535;
h) J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell, H.
Niu, C. Dickinson, A. Y. Ganin, M. J. Rosseinsky, Y. Z. Khi-
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7483
.
Angewandte
Communications
myak, A. I. Cooper, Angew. Chem. 2007, 119, 8728 – 8732;
Angew. Chem. Int. Ed. 2007, 46, 8574 – 8578.
[9] W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna, H.-C. Zhou, J.
Am. Chem. Soc. 2011, 133, 18126 – 18129.
[10] J. Schmidt, M. Werner, A. Thomas, Macromolecules 2009, 42,
4426 – 4429.
[11] L. Wang, R. T. Yang, J. Phys. Chem. C 2012, 116, 1099 – 1106.
7484
www.angewandte.org
[12] M. G. Rabbani, H. M. El-Kaderi, Chem. Mater. 2012, 24, 1511 –
1517.
[13] C. Pevida, T. C. Drage, C. E. Snape, Carbon 2008, 46, 1464 –
1474.
[14] J. A. Mason, K. Sumida, Z. R. Herm, R. Krishna, J. R. Long,
Energy Environ. Sci. 2011, 4, 3030 – 3040.
[15] A. L. Myers, J. M. Prausnitz, AIChE J. 1965, 11, 121 – 127.
2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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